I don't understand what you really gain with this setup. I mean, this extreme vendor lock-in situation is so short term. The absolute wrong strategy if you ask me. I would be curious to see this company in 5 years from now.

Let me ask you this question differently: lets say you are exploring a new market opportunity for the an exciting product you want to build. Would you rather spend cycles building what AWS has done already and thus delay speed to market or would you rather use what has already been done by AWS managed services and build what nobody has done before? Surely, this architecture will evolve over time, but it will only evolve as the startup quickly discovers what the market needs.